1. Field of the Invention
The present invention relates to a semiconductor laser for use in the fields of optical communication systems, such as optical local area (LAN) networks, optical information processing, optical recording and the like, a modulation method for modulating or driving the semiconductor laser and an optical communication system using the semiconductor laser as a transmitter, for example. More in particular, the present invention relates to an oscillation polarization mode selective semiconductor laser whose oscillation polarization mode can be switched between mutually-perpendicular polarization modes and which can suppress dynamic wavelength fluctuation even during high-speed modulation time and stably achieve high-density frequency or wavelength division multiplexing (FDM or WDM) optical communication and the like.
2. Related Background Art
In recent years, increased transmission capacity in the field of optical communications has become desirable, and the development of optical frequency or wavelength division multiplex communication, in which signals at a plurality of optical frequencies or wavelengths are multiplexed in a single optical fiber, has been advanced.
It is important to narrow the wavelength or frequency interval between multiplexed signals to increase the transmission capacity as much as possible. For this purpose, it is desirable that the transmission bandwidth of a wavelength tunable filter or demultiplexer is small and the occupied frequency bandwidth or spectral line width of a laser used as a light source is narrow. In a distributed feedback (DFB) filter whose tunable bandwidth is 3 nm, for example, since the transmission bandwidth is about 0.03 nm, multiplexing of 100 channels is possible. In this case, however, the spectral line width is required to be less than 0.03 nm. At present, even the spectral line width of a DFB laser, which is also known as a dynamic single mode semiconductor laser, is widened to about 0.3 nm due to the dynamic wavelength fluctuation when a direct amplitude shift keying (ASK) modulation is performed to the single mode semiconductor laser. Therefore, this kind of laser is unsuitable for the FDM or WDM transmission.
Therefore, to reduce such wavelength fluctuation, a system using an external intensity modulator (see Suzuki et al., ".lambda./4 Shift DFB Laser/Light Source Integrated With Absorption Type Optical Modulator", Report in Symp. of Japan Electronics Information Communication Academy, OQE 90-45, p. 99, 1990, for example), a direct FSK modulation system (see M. J. Chawski et al. "1.5 Gbit/s FSK Transmission System Using Two Electrode DFB Laser As A Tunable FSK Discriminator/Photodetector", Electron. Lett. Vol. 26 No. 15, 1990, for example), a direct polarization modulation system (see Japanese Patent Laid-open No. 2 (Heisei)-159781, for example) have been proposed.
In an external intensity modulator, wavelength fluctuation is about 0.03 nm, which is the marginal value that can satisfy the required spectral linewidth, and the number of devices is high, leading to an increase in cost. In an FSK system, a filter on the receiver side is required to function as a wavelength discriminator and thus, sophisticated control techniques are needed.
However, in a direct polarization modulation system, only a plural number of electrodes of an ordinary DFB laser are needs. Also, the number of devices is less, and the wavelength fluctuation is smaller, than that of an external modulation system. Further, receiver filter requirements are less demanding since the transmission signal is ASK.
As described above, a polarization modulation system is a modulation system suitable for wavelength division multiplexing transmission and the like. In that system, however, a positive means for enabling the polarization-mode switching is not disclosed. Therefore, a highly reproducible device is difficult to fabricate, and adjustment is needed after its fabrication, leading to a poor yield.